Lead-free Cu-Zn alloy

ABSTRACT

A lead-free Cu—Zn base alloy consisting of: 58-64 wt % Cu; 0.4-1.4 wt % Fe; 0.4-2.3 wt % Mn; 1.5-3.5 wt % Ni; 0.1-4.4 wt % Al; 0.5-1.8 wt % Si; as an alloy component that promotes chip breaking either 0.65-1.2 wt % Sn with up to 0.025 wt % P, or 0.03-0.1 wt % P with up to 0.25 wt % Sn; up to 0.1 wt % Pb; balance Zn together with unavoidable impurities, which are permitted up to 0.05 wt % per element, wherein the sum total of unavoidable impurities does not exceed 0.15 wt %; and wherein Cr is tolerated up to 0.035 wt %.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to EP 20204604.1 filed Oct. 29, 2020 which is incorporated by reference herein.

BACKGROUND

The present disclosure relates to a lead-free Cu—Zn base alloy with good machining properties.

Due to the small number of elements involved in the structure of the alloy, the alloy CuZn42 is a very simply structured brass alloy with a Cu content between 57.0 and 59.0 wt %. In principle, no other elements are involved in this alloy. Pb is tolerated up to a maximum of 0.2 wt %, Sn up to 0.03 wt %, Fe up to 0.3 wt %, Ni up to 0.02 wt % and Al up to 0.05 wt % together with unavoidable impurities. This alloy is a lead-free alloy that can be hot-worked very easily and is used, among other things, for the production of profiles as a semi-finished product. This alloy is the lead-free variant of the conventionally used alloy CuZn39Pb3. In the case of the CuZn39Pb3 alloy, the element lead is used primarily to improve the machinability. Even though the alloy CuZn42 is lead-free, it is also used for machining, such as the production of turned parts, due to its α/β microstructure. However, the machinability of workpieces made from this alloy is limited. This means that the machining disadvantages caused by the alloy cannot be compensated by appropriate process parameters of a machine tool. This applies, for example, to machining processes with forming tools, in which the limits of the process parameters do not allow any respective leeway. In such cases, the machinability of such an alloy is unsatisfactory.

Even if the machinability for certain machining operations on workpieces made from this alloy is acceptable, it would be desirable if the machinability could be improved without having to use the Pb and Bi elements conventionally used for free cutting alloys to achieve the desired machinability, since these are classified as hazardous to health.

The above applies likewise to special brass alloys, which are optimized for specific properties by involving other elements, such as Fe, Mn, Ni, Al and/or Si. The alloys CuZn28Al4Ni3Si1Mn and CuZn35Mn2Ni2FeSi can be given here as examples. The first alloy mentioned is characterized by high wear resistance and high strength with good running and sliding properties. This alloy and workpieces made therefrom are therefore suitable for applications with oil lubrication under boundary friction conditions. This alloy is also suitable for use in bio-lubricants. The second alloy mentioned is particularly suitable for bearing and sliding applications, particularly also for bearing shafts or journals made of aluminum materials. In this special brass alloy, special attention was paid to its resistance to corrosion.

A Cu—Zn alloy with improved machining properties is known from EP 3 690 069 Cl. This alloy contains 58-70 wt % Cu, 0.5-2.0 wt % Sn, 0.1-2.0 wt % Si, the balance being zinc and unavoidable impurities, with the sum total of the elements Sn and Si between 1.0 wt % and 3.0 wt %. The improved machinability without the use of the elements Pb and Bi is provided in this alloy by the Sn and Si contents. In the specified proportions, these elements are responsible for the formation of the £ phase, which phase is distributed as a microstructure in the alloy and thus promotes chip breaking. The Si contained in the alloy also leads to the formation of silicides, specifically together with the elements Al and Ni permitted in the alloy and/or Mn, which are regularly found in the alloy due to the usual use of recycled material. The Si content in this alloy can be 2.0 wt %. It is true that silicides contained in the matrix are advantageous for some uses, especially when there are requirements for wear resistance.

JP S56-127741 A discloses a Cu—Zn alloy with improved abrasion resistance under more extreme sliding conditions. This alloy has a composition of 54 to 66 wt % Cu, 1.0 to 5.0 wt % Al, 1.0 to 5.0 wt % Mn, 0.2 to 1.5 wt % Si, 0.5 to 4.0 wt % Ni, 0.1 to 2.0 wt % Fe, 0.2 to 2.0 wt % Sn, the balance being Zn together with unavoidable impurities. This alloy has a special Ni and Fe content to form manganese silicides. The Sn content is used to improve the strength and toughness of the raw product. No other elements are permitted in this alloy. No information is given on the machining behavior of this alloy.

WO 2015/117972 A2 discloses a lubricant-compatible copper alloy having the following composition in percent by weight: 54 to 65% Cu, 2.5 to 5.0% Al, 1.0 to 3.0% Si, 2.0 to 4.0% Ni, 0.1 to 1.5% Fe, up to 1.5% Mn, up to 1.5% Sn, up to 1.5% Cr, up to 0.8% Pb, the balance being Zn together with unavoidable impurities. The composition of this Cu—Zn alloy is selected so that at least 0.4% free Si is present. This silicon, which is not bound in silicides, can also be contained in silicon-containing non-silicide phases. No other elements are permitted in this alloy as well. In this alloy, the focus is on the use of a component made therefrom in an oil environment, e.g. a synchronizer ring, through the use of different oils in different tribological systems. Information on machinability is not given with respect to this alloy.

DE 10 2017 007 138 B3 relates to material for aquaculture, specifically a wire material and a net made therefrom or a breeding cage made therefrom. Information on the machining behavior is understandably not given due to the intended use. The exemplary embodiments described in this document have the following composition: 63.8 to 65.5 wt % Cu, 0.9 to 1.1 wt % Sn, 0.2 to 0.3 wt % Fe, 0.15 to 0.2 wt % P, the balance being Zn and, in one embodiment, 0.6 wt % Al. With this composition, the wire material should have a first oxide layer, which partially covers the metallic material, and a second oxide layer, which covers the metallic materials in those areas that are not covered by the first oxide layer, for aquaculture resistance.

SUMMARY

Proceeding from this background, one aspect of the present disclosure is to provide a lead-free Cu—Zn alloy with improved machining properties, especially one that can be used as a base alloy and that does not require any special manufacturing steps to produce the desired machining properties.

This is achieved by a lead-free Cu—Zn base alloy consisting of (data provided in wt %):

-   -   Cu: 58-64%,     -   Fe: 0.4-1.4%,     -   Mn: 0.4-2.3%,     -   Ni: 1.5-3.5%,     -   Al: 0.1-4.4%,     -   Si: 0.5-1.8%,     -   alloy component that promotes chip breaking:     -   Sn 0.65-1.2%, wherein P is involved in the structure of the         alloy up to a maximum of 0.025%, or     -   P 0.03-0.1%, wherein Sn involvement up to a maximum of 0.25% is         tolerated,     -   the balance is made up by Zn together with unavoidable         impurities, which are permitted up to 0.05% per element, wherein         the sum total of the unavoidable impurities does not exceed         0.15%,     -   Pb: up to 0.1%,     -   wherein Cr with up to 0.035% is tolerated.

In the context of these explanations, an alloy is referred to as a base alloy in which alloy products with different properties can be provided by varying the alloying elements in one and the same manufacturing process. Such an alloy has the advantage that contamination is minimized when the alloy is melted if alloy products with different properties are to be manufactured.

Unavoidable impurities are permitted at 0.05 wt % per element, whereby the sum total of the unavoidable impurities does not exceed 0.15 wt %.

An alloy is considered to be lead-free if its Pb content does not exceed 0.1 wt %.

This alloy may contain P to achieve the desired improved machining properties. If the alloy contains P, manganese and iron phosphides are formed at the grain boundaries, which significantly improves the machinability compared to the alloys CuZn28Al4Ni3Si1Mn and CuZn35Mn2Ni2FeSi. Since the alloy also contains Si, silicides are also formed in the matrix, typically with the involvement of the elements Fe, Mn, also Ni and Al. The silicides contribute to wear resistance, but together with the phosphides they also promote machinability. The grain sizes of the phosphides and the silicides are relatively small, such that tool wear can be kept small during machining.

If P is contained as an alloy component that promotes chip breaking, it is contained in proportions between 0.03-0.1 wt %. The P content is thus limited to 0.1 wt %. At higher P contents, coarser phosphides are formed, which in turn is disadvantageous for machinability and surface processing, such as polishing or coating of the workpiece surface after the machining process. This disadvantage is not compensated by the improved wear resistance if the focus of the alloy produced is on optimized machinability. The Cu—Zn base alloy containing P is a first variant of the alloy according to the present disclosure.

According to a second variant of this alloy, as an alloy component that promotes chip breaking, Sn is involved in the structure of the alloy in proportions between 0.65 and 1.2 wt %. Sn is incorporated into the mixed crystal below the solubility limit. The Sn content in this alloy is limited to 1.2 wt %, since otherwise there is a risk that Sn-containing y phases could form which have an embrittling effect. Sn increases work hardening and strength and thus has a beneficial effect on chip breaking and thus on the machinability of a workpiece made from this alloy. In addition, Sn tends to form Sn oxides during dry machining, which oxides are transferred to the tool surface and thereby reduce tool wear. The Sn content preferably matches the Fe content±20%.

In one embodiment, P and Sn are jointly involved in the construction of the alloy. When using P, it is advantageous that the melt solidifies in a fine-grained manner. However, P has the disadvantage that it makes the melt less viscous. Sn counteracts this aspect, but without adversely affecting the positive structure-forming properties of P in the melt. Sn can also have a deoxidizing effect in the melt, which is an advantage for the alloy, both with P and without P.

Broken chips when machining a workpiece made from this alloy usually have the desired chip shape (crumbling chips or very short helical chips). The chip shape thus corresponds to that which is found in a machining operation of the CuZn39Pb3 alloy, which is regarded as particularly good for machining.

Surprisingly, it was found that in this alloy, the phosphides act as oxidation inhibitors of the structure, particularly at elevated temperatures.

The elements Fe and Mn are limited to the stated contents. If more Fe or Mn is used, this results in coarsening of the grain. Below the limits mentioned, the desired phosphides do not develop to a sufficient extent to achieve the machining-improving properties.

The tolerated accompanying elements do not adversely affect the improved machinability of a workpiece made from the alloy according to the present disclosure, at least not significantly. Recycled material can therefore be used to produce this alloy without having to accept disadvantages. For this purpose, recycling material from a preferably closed cycle is used, i.e. single-type recycling material is used. If recycling material is used in which, with regard to its composition, for example, one or more elements are not present or not in the appropriate proportion, these elements can be added to the recycling material. This particularly applies to the element P, which is generally not present when using conventional recycling material.

A special feature of the alloy according to the present disclosure is that the improved machinability is based solely on the special composition of the alloy and that no additional measures, such as certain manufacturing or processing steps, are required. Therefore, the semi-finished products (workpieces) produced from the alloy can be produced using the usual manufacturing processes. This also has the advantage that for the processing of the semi-finished products to manufacture the final product, respective treatment steps to set specific strength and/or structural properties can be performed; these are therefore not yet consumed by the manufacturing process for producing the semi-finished products. In this context, it goes without saying that the improved machining properties are achieved without additional process steps, but that, if desired, they can be increased again after extrusion through special treatment steps. The machining properties can be improved, for example, by cold deformation and the associated work hardening, since this improves chip breaking and thus machinability. This can be followed by stress relief annealing to reduce internal stresses. Such a process step can also be used to influence the microstructure, for example to set an α/β microstructure as fine and heterogeneous as possible or to generate precipitation phases, such as very fine silicides or α-precipitates in a β-matrix.

A first variant of the alloy according to the present disclosure has the following contents of the specified elements:

-   -   Cu: 59-63 wt %, particularly 59.5-61 wt %     -   Fe: 0.4-1.1 wt %, particularly 0.6-1.1 wt %     -   Mn: 0.4-1.1 wt %, particularly 0.6-1.0 wt %     -   Ni: 2.5-3.7 wt %, particularly 2.6-3.3 wt %     -   Al: 3.3-4.2 wt %, particularly 3.5-4.1 wt %     -   Si: 1.0-1.8 wt %, particularly 1.1-1.7 wt %

This variant is designed for high strength values in the workpiece. This alloy therefore has a relatively high Si content and higher contents of the elements Ni and Al.

A second variant of the alloy contains the following elements in the specified proportions:

-   -   Cu: 60-62.5 wt %     -   Fe: 0.8-1.4 wt %, particularly 0.85-1.25 wt %     -   Mn: 1.4-2.3 wt %, particularly 1.5-2.1 wt %     -   Ni: 1.5-2.5 wt %, particularly 1.7-2.35 wt %     -   Al: 0.1-0.7 wt %, particularly 0.2-0.5 wt %     -   Si: 0.5-1.2 wt %, particularly 0.6-1.0 wt %

Ultimately using the same alloying elements as the first variant, this alloy is significantly softer and has lower strength properties and is therefore suitable for other applications.

These two variants already illustrate the range of the base alloy according to the present disclosure, wherein workpieces with different strength properties can be produced simply by varying the elements and without having to change the manufacturing process.

The variation of the elements also has an effect on the structure, since these have different zinc equivalents and thus both workpieces with predominantly β-phase and workpieces with a structure of α-phase with embedded β-phase can be produced with this alloy. While the former are characterized by good hot formability, depending on the α-component also by a specific cold formability, the latter is more suitable for cold forming.

DESCRIPTION OF FIGURE

FIG. 1 is a compositional table for alloy specimens.

DESCRIPTION OF TESTS

Specimens from the alloys listed in the table of FIG. 1 were shaped into bars by continuous casting and subsequent extrusion, then straightened and then subsequently thermally relaxed. Alloys 1 to 6 are alloys according to the present disclosure, wherein alloys 1 to 3 belong to the first variant and alloys 4 to 6 belong to the second variant.

Specimens were cut off from the semi-finished products produced with a cylindrical outer surface. The machining tests were carried out uniformly for all specimens by external longitudinal turning at a cutting speed of 200 m/min, made with a depth of cut of 1 mm and a feed of 0.1 mm.

The results of the tests were rated in the form of indices from 0 to 100. In this system, the comparison alloy CuZn42 receives the index 50 for the various cutting indexes. The higher the index, the better the result.

Chip shape, cutting force, tool wear, and surface quality resulting from the cutting were examined. The results of the tests are given in the following table:

Chip Cutting Tool Surface shape force wear finish CuZn39Pb3 80 40 60 70 CuZn42 50 50 50 50 CuZn35Mn2Si 80 40 60 70 CuZn28Al4Ni3Si1Mn 60 25 50 60 CuZn35Mn2Ni2FeSi 50 40 60 70 Alloy 1 70 30 55 70 Alloy 2 70 40 60 70 Alloy 3 70 30 55 70 Alloy 4 70 50 60 65 Alloy 5 70 50 50 70 Alloy 6 70 55 60 65

A somewhat higher cutting force is required for cutting the alloys according to the present disclosure. The reason for this is the phosphides contained in the alloy, which, however, are responsible for better chip breaking and thus also for the overall improved machinability. Regarding machinability, the chip shape is a relevant factor, such that in this regard the somewhat higher cutting force compared to the Pb-containing comparison alloys can be accepted.

It is important to note that the alloys according to the present disclosure have an improved tool wear index compared to the CuZn42 alloy. This was not expected.

The surface quality of the alloys according to the present disclosure substantially corresponds to that which is achieved with the comparison alloys, such that no disadvantages, at least no noteworthy disadvantages, have to be accepted in this respect.

The mechanical strength values of the alloys according to the present disclosure produced with the above-described process: continuous casting, extrusion, straightening, thermal stress relief are shown in the following table as an example for alloys 1 and 4 and compared to the strength values of comparison alloys:

Yield Tensile Brinell strength Rp strength Elongation hardness 0.2 [MPa] [MPa] at break % HBW CuZn35Mn2Si 223 463 32.8 118 CuZn28Al4Ni3Si1Mn 598 811 11 255 Alloy 1 530 570 10 260 CuZn35Mn2Ni2FeSi 177 417 34 107 Alloy 4 210 430 32 120

The mechanical characteristics listed in the table above of the alloys according to the present disclosure in comparison with reference alloys make it clear that the improved machinability of the alloys according to the present disclosure does not have a disadvantageous effect on the mechanical strength values. In principle, these match the values of the respective reference alloy.

Due to the positive structural properties that are already established during pressing, a semi-finished product made from the alloy can be used for a wide variety of applications.

While a number of aspects and embodiments have been discussed herein, those skilled in the art will recognize numerous modifications, permutations, additions, combinations and sub-combinations therefor, without same needing to be specifically explained in the context of this disclosure. The claims should therefore be interpreted to include all such modifications, permutations, additions and sub-combinations, which are within their true spirit and scope. The terms and expressions herein are used for description and not limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown or described, or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the invention has been specifically disclosed by certain embodiments and optional features, modification and variation of the concepts herein may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the claims. Whenever a range is given, all intermediate ranges and subranges, as well as all individual values included in the ranges given are hereby incorporated into this disclosure. When a Markush group or other grouping is used, all individual members of the group and all combinations and sub-combinations possible of the group are hereby individually included in this disclosure. In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, references and contexts known to those skilled in the art. The above definitions are provided to clarify their specific use in the context of the invention. 

1. A lead-free Cu—Zn base alloy consisting of: Cu: 58-64 wt %; Fe: 0.4-1.4 wt %; Mn: 0.4-2.3 wt %; Ni: 1.5-3.5 wt %; Al: 0.1-4.4 wt %; Si: 0.5-1.8 wt %; an alloy component that promotes chip breaking either: Sn: 0.65-1.2 wt %, wherein P is involved up to 0.025 wt %, or P: 0.03-0.1 wt %, wherein Sn involvement up to 0.25 wt % is tolerated; Pb: up to 0.1 wt %; balance Zn together with unavoidable impurities, which are permitted up to 0.05 wt % per element, wherein the sum total of unavoidable impurities does not exceed 0.15 wt %; wherein Cr is tolerated up to 0.035 wt %.
 2. The Cu—Zn base alloy of claim 1, containing: Cu: 59.5-61 wt %; Fe: 0.4-1.1 wt %; Mn: 0.4-1.1 wt %; Ni: 2.5-3.7 wt %; Al: 3.3-4.2 wt %; Si: 1.0-1.8 wt %.
 3. The Cu—Zn base alloy of claim 2, containing: Fe: 0.6-1.1 wt %; Mn: 0.6-1.0 wt %; Ni: 2.6-3.3 wt %; Al: 3.5-4.1 wt %; Si: 1.1-1.7 wt %.
 4. The Cu—Zn base alloy of claim 2, wherein the alloy contains 0.03-0.1 wt % P and Sn is tolerated up to 0.25 wt %.
 5. The Cu—Zn base alloy of claim 4, wherein the alloy contains 0.05-0.08 wt % P.
 6. The Cu—Zn base alloy of claim 2, wherein the alloy contains 0.65-1.2 wt % Sn and P is tolerated up to 0.02 wt %.
 7. The Cu—Zn base alloy of claim 6, wherein the alloy contains 0.7-1.1 wt % Sn.
 8. The Cu—Zn base alloy of claim 1, containing: Cu: 60-62.5 wt %; Fe: 0.8-1.4 wt %; Mn: 1.4-2.3 wt %; Ni: 1.5-2.5 wt %; Al: 0.1-0.7 wt %; Si: 0.5-1.2 wt %.
 9. The Cu—Zn base alloy of claim 8, containing: Fe: 0.85-1.25 wt %; Mn: 1.5-2.1 wt %; Ni: 1.7-2.35 wt %; Al: 0.2-0.5 wt %; Si: 0.6-1.0 wt %.
 10. The Cu—Zn base alloy of claim 8, wherein the alloy contains 0.03-0.1 wt % P and Sn is tolerated up to 0.25 wt %.
 11. The Cu—Zn base alloy of claim 10, wherein the alloy contains 0.05-0.08 wt % P.
 12. The Cu—Zn base alloy of claim 8, wherein the alloy contains 0.65-1.2 wt % Sn and P is tolerated up to 0.02 wt %.
 13. The Cu—Zn base alloy of claim 12, wherein the alloy contains 0.7-1.1 wt % Sn. 